The present invention relates to an injector drive circuit.
Conventional internal combustion engine control devices for automobiles, motorcycles, agricultural machines, machine tools and ship machinery using gasoline and light oil as fuel have injectors that directly inject fuel into cylinders to improve fuel consumption and output. Such injectors are called “in-cylinder direct injection type injectors”, “direct injector” or “DI”.
Current mainstream gasoline engines employ a port injection system that injects fuel into an intake manifold. An engine with the in-cylinder direct injection type injectors using highly pressurized fuel requires higher energy during an injector valve opening operation than does the port injection system. To improve controllability to cope with faster revolutions, high energy must be supplied to the injectors in a short period of time. Further, in engines with the in-cylinder direct injection type injectors, attention is being focused on a technology called a multiple injection which is designed to reduce fuel cost and exhaust emissions. This technology, however, is required to supply high energy to the injectors in an even shorter period of time because the same amount of fuel that is injected once in one stroke of the conventional piston needs to be injected in several divided portions at different timings.
Many injector drive circuits to control the in-cylinder direct injection type injectors generally have a step-up circuit that boosts a battery voltage to a higher voltage that is applied to the injectors to reduce their response time. So, in the multiple injection technology that has an increased number of injector operations, a burden on the step-up circuit increases, making it an important issue to reduce the load of the step-up circuit.
Now, a typical current waveform of the direct injector will be explained. First, during a peak current application period at an initial stage of injector energization, the injector current is raised to a predetermined peak current in a short period of time using a stepped-up voltage to open an injector valve. This peak current, when compared with the injector current in the system that injects fuel into an intake manifold, is about 5-20 times higher. After the peak current application period ends, the source of energy supply to the injector changes from the step-up circuit to the battery power, supplying a lower current than the peak current to keep the injector valve open. By supplying the peak current and the valve open state holding current, the open injector injects fuel into the cylinder.
At the end of fuel injection, the injector current must be cut off to quickly close the injector valve by lowering the injector-energizing current in a short time. The injector, however, has high energy stored therein by the injector current flowing through it. So, it is necessary to eliminate this energy from the injector. To accomplish this in a short period of time, various kinds of methods are used, including one which transforms the energy into thermal energy by a switching device in an injector current application circuit utilizing a Zener diode effect and one which, through a current regeneration diode, regenerates the injector current to a step-up capacitor that stores the boosted voltage from the step-up circuit.
JP-A-2008-169762, for example, discloses a technology that controls a current flowing through the injector by simultaneously energizing the step-up circuit and the battery drive circuit, both as energy supply sources.